RXTE Studies of Accreting X-ray Pulsars via Cyclotron Lines
Rick Rothschild on behalf of the MAGNET collaboration
• RXTE has doubled the number of known cyclotron lines sources
• RXTE has observed multiple line sources in outburst
• RXTE has provided multiple visits to both persistent and transient sources to allowluminosity-dependent effects to be investigated
• Spin phase resolved spectroscopy has allowed for probing different portions of the scattering region as the neutron star rotates.
• RXTE has revealed variations with pulse shape.
Heindl et al. 2003
The Numbers:
68 accreting X-ray pulsars25 Be binaries 6 Possible Be binaries14 Supergiants10 Symbiotics binaries3 Low mass X-ray Binaries1 High Mass X-ray binary 1 Intermediate mass (Her X-1)1 Possible white dwarf7 Unknown Companion
17 Sources exhibiting one or more cyclotron lines
9 Be binaries5 Supergiants1 Intermediate mass1 Symbiotic binary (New)1 Possible white dwarf
5 Sources possibly having a line3 Be binaries1 Supergiant1 Unknown
Caballero et al. 2010, 2011
A0535+26
RXTE Observations of Cyclotron Line Variability with Luminosity
Theory: Emission Geometry
Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars
Theory: Variation of CRSF Energy
Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars
• We see 4 distinct types of variation of Ecyc with variation of Lx
Supercritical,radiationdeceleration
Lx
Subcritical,Coulomb stopping
Subcritical,gas-shock
Subcritical,direct impact
Ecyc
Ecyc = E*
Lx
Region of Interest
Theory: Variation of Emission Height
Critical Luminosity for Bimodal Spectral Variabilityin Accretion-Powered X-Ray Pulsars
• The observations of Ecyc imply 4 distinct types of variation of the characteristic emission height h with variation of Lx
Supercritical,radiationdeceleration
Lx
h
Subcritical,Coulomb stopping
Subcritical,gas-shock
Subcritical,direct impact
h = 0
Lx
Advances in Understanding Cyclotron Lines from a Physics Point of View
Predicted cyclotron line shapes on top of a Fermi-Dirac continuum model with Ecut=10 keV and Efold=12 keV. Modeled is a cylindrical column with B/Bcrit = 0.05, kTe = 3 keV, τes = 10-3. Note Bcrit = 4.4 x 1013 Gauss
Combining the Continuum and Cyclotron Line Modeling
Her X-1, the Original Cyclotron Line Source
Trümper et al. 1977
35 day super modulation
1.24 s pulsed flux
The evolution of the pulse profile is intimately linked to the evolution of the 35 day Main-On.
The left shoulder goes away as the Main-On progresses.
Klochkov & Staubert
Variation of Power Law Photon Index with Pulse Phase and Main-On Interval
The power law photon index softens slightly with pulse phase as the pulse profile varies throughout the 35 day Main-ON, with two EXCEPTIONS:
1) Phase ~0.6: Abrupt change in index lessens as the left shoulder of the profile diminishes.
2) Phase ~0.7-0.9: The index flattens from 0.7-0.8 and steepens again from 0.8-0.9
Is this Steve Pravdo’s viewing down the pole towards us and the gravitationally beamed pole on the back side?
While the power law photon index varies with pulse shape in the Main-On, the cyclotron line variation is essentially the same throughout the Main-On
Thus our view of the cyclotron resonant scattering region is unchanged throughout the 35-day Main-On, but the continuum emission region is affected.
Variation of the Cyclotron Line with 35 Day Phase
Behavior is the same for different 35 Day Main-On’s:
Apparent slow rise in centroid energy and then abrupt decrease.
What is the long term cyclotron line variation in Her X-1 telling us?
• The abrupt change in the 1980’s
• The slow decrease since the 1990’s
Variation of Her X-1 Cyclotron Line Energy with Mean Main-On Flux
Staubert & Klochkov
Red Points are Post-2006
Black Points are Pre-2006
Variation of Cyclotron Energy with Average Main-On Flux
This works well until about 2007, then another possible abrupt change occurs or the relationship no longer holds.
Luminosity (i.e. ASM Main-On Maximum) Corrected Cyclotron Line Energies up to ~2007, and uncorrected after that.
Yet again, Her X-1 is presenting us with surprises and challenges to our understanding of magnetic poles of neutron stars.
Thank you to everyone who made RXTE a big success.
Present List of Cyclotron Line Sources
Possible new cyclotron line source: IGR J18179-1621 with 11.8 s period and Ecyc~21 keV
Becker & Wolff (2007) and subsequent work by Becker and his students are developing a bulk Comptonization model of accretion onto the magnetic poles.
Eventually this physics-based treatment will replace empirical modeling of accretion powered pulsar spectra.
